Most automotive air bag restraint systems, presently in use, use gas generant compositions in which sodium azide is the principal fuel. Because of disadvantages with sodium azide, particularly instability in the presence of metallic impurities and toxicity, which presents a disposal problem for unfired gas generators, there is a desire to develop non-azide gas generant systems, and a number of non-azide formulations have been proposed. However, to date, non-azide gas generants have not made significant commercial inroads.
Alternatives to azides which have been proposed, e.g., in U.S. Pat. No. 5,035,757, the teachings of which are incorporated herein by reference, include azole compounds, particularly tetrazole and triazole compounds. Tetrazole compounds include, for example, 5-amino tetrazole (5-AT), tetrazole, and bitetrazole. Triazole compounds include, for example, 1,2,4-triazole-5-one, and 3-nitro 1,2,4-triazole-5-one. Although all of the above azole compounds are useful fuels in accordance with the present invention, 5-AT is the most commercially important of these.
Gas generant systems include, in addition to the fuel component, an oxidizer component. Proposed oxidizers for use in conjunction with azole fuels include alkali and alkaline earth metal salts of nitrates, chlorates and perchlorates. Another type of oxidizer for tetrazoles and triazoles, as taught, for example, in U.S. Pat. No. 3,468,730, the teachings of which are incorporated herein by reference, are metal oxides, particularly transition metal oxides. Transition metal oxides suitable as oxidizers include, but are not limited to cupric oxide, ferric oxide, lead dioxide, manganese dioxide and mixtures thereof. Metal oxides are desired as oxidizers in that they tend to lower combustion temperatures, thereby lowering the generated levels of toxic oxides, such as CO and NO.sub.x.
Several gas generant processing procedures utilize water. Water-processing reduces hazards of processing gas generant materials. It is therefore desirable that gas generant compositions be formulated so as to facilitate water processing.
One Example of water processing, taught, e.g., in U.S. Pat. No. 5,015,309, the teachings of which are incorporated by reference, involves the steps of
1. Forming a slurry of the generant ingredients with water. PA1 2. Spray drying the slurry to form spherical prills of diameter 100-300 microns. PA1 3. Feeding the prills via gravity flow to a high speed rotary press. PA1 1. Forming a slurry of the generant ingredients with water. PA1 2. Extruding the slurry to form spaghetti like strands. PA1 3. Chopping and spheronizing the strands into prills. PA1 4. Tableting of the prills as described previously. PA1 The compound should be readily soluble in water, i.e., at least about 30 gm/100 ml. H.sub.2 O at 25.degree. C.; PA1 The compound should contain only elements selected from H, C, O and N; PA1 When formulated with an oxidizer to stoichiometrically yield carbon dioxide, nitrogen, and water, the gas yield should be greater than about 1.8 moles of gas per 100 grams of formulation; and PA1 When formulated with an oxidizer to stoichiometrically yield carbon dioxide, water and nitrogen, the theoretical chamber temperature at 1000 psi should be low, preferably, less than about 1800.degree. K.
Another common production technique, (e.g. U.S. Pat. No. 5,084,218), the teachings of which are incorporated herein by reference, involves the following steps:
A problem has been found with gas generant compositions containing both a triazole and/or a tetrazole having an acidic hydrogen plus a metal oxide oxidizer, a problem particularly seen if the composition is aqueous-processed, is poor long-term stability (as demonstrated by accelerated heat-aging experiments). Over time, the amount of the fuel is found to decrease and the performance decreases. Thus, if such a gas generant were used in an automotive airbag inflator, the inflator, over time, might become insufficiently effective. While Applicants are not bound by theory, it is believed that the metal ion of the metal oxide replaces, over time, acidic hydrogens of tetrazoles and/or triazoles, producing metal salts or complexes. These metal salts or complexes are somewhat unstable and, over time, decompose.
It is a primary object of the invention to stabilize gas generant compositions containing tetrazoles and/or triazoles having an acidic hydrogen plus a transition metal oxide oxidizer.